This invention relates to a process for selectively etching silicon, a portion of the surface of which is masked.
Silicon is the most useful electronic material. However, dry etchants for silicon are quite limited in their capabilities. For example, fluorinated gases can etch silicon faster than silicon dioxide, the latter being a typical mask material. However, this etching is isotropic (non-directional) as is wet chemical etching. Directional etching of deep structures in silicon is often made difficult by high mask erosion rates. For directional etching, erosion rate ratios of about 8 (Si/SiO.sub.2) have been the best obtainable. Chlorinated gases etch silicon at an inherently higher rate and a rate ratio of about 20 has been obtained for Si/SiO.sub.2 in relatively high pressure reactive sputter etchings. It is possible to further manipulate chamber pressure and target voltage for a yet higher ratio, but etch rate falls and etch quality can suffer from micro-roughness on the surface of the silicon, known as "black plague".
It has been proposed in U.S. Pat. No. 4,213,818 to provide a selective vapor etching process wherein a gas plasma is used to preferentially etch silicon with respect to silicon dioxide masking material. The gas plasma is formed utilizing a radio frequency generator which is not attached to the target silicon. Unfortunately, the silicon to silicon dioxide etching rate ratios are unsatisfactorily low, and the etching is not directional.